Tropomyosin Related Kinases (Trk) are a family of receptor tyrosine kinases composed of three family members, TrkA, TrkB and TrkC. The Trks bind with high affinity to, and mediate the signal transduction induced by the Neurotrophin family of ligands whose prototype members are Nerve Growth Factor (NGF), Brain-Derived Neurotrophic Factor (BDNF) and Neurotrophin-3, -4 and -5 (NT-3, NT-4 and NT-5). In addition, a co-receptor lacking enzymatic activity, p75, has been identified which binds all neurotrophines (NTs) with low affinity and regulates neurotrophin signaling. A critical role of the Trks and their ligands during the development of the central and peripheral nervous systems have been established through gene disruption studies in mice. In particular, TrkA-NGF interaction was shown as a requirement for the survival of certain peripheral neuron populations involved in mediating pain signaling. In addition to these developmental consequences of Trk signaling, the subversion of this receptor and its signaling pathway in certain malignancies has also been documented. Of particular note are reports of aberrant expression of NGF and TrkA receptor kinase are implicated in the development and progression of human prostatic carcinoma and pancreatic ductal adrenocarcinoma and activating chromosomal rearrangements of Trks in acute myelogenous leukemia (AML), thyroid and breast cancers and receptor point mutations predicted to be constitutively activating in colon tumors. In addition to these activation mechanisms, elevated Trk receptor and ligand have also been reported in a variety of tumor types including multiple myeloma, melanoma, neuroblastoma, ovarian and pancreatic carcinoma. The neurotrophins and their corresponding Trk receptor subtypes have been shown to exert a variety of pleiotropic responses on malignant cells, including enhanced tumor invasiveness and chemotaxis, activation of apoptosis, stimulation of clonal growth, and altered cell morphology. These effects have been observed in carcinomas of the prostate, breast, thyroid, colon, malignant melanomas, lung carcinomas, glioblastomas, pancreatic carcinoids and a wide variety of pediatric and neuroectodermal-derived tumors including Wilm's tumor, neuroblastomas and medulloblastomas. Neurotrophins and their receptor subtypes have been implicated in these cancers either through autocrine or paracrine mechanisms involving carcinoma cells and the surrounding parenchymal and stromal tissues. In addition, profound or significantly attenuated reduction of bone pain caused by prostate cancer metastasis has recently been achieved by utilization of an anti-NGF antibody. Overall, the oncogenic properties of Trk signaling in multiple tumor types makes the modulation of the Trk receptor signaling a potentially attractive therapeutic intervention point in different malignancies.
Receptor tyrosine kinases (RTKs) are important in the transmission of biochemical signals across the plasma membrane of cells. These transmembrane molecules characteristically consist of an extracellular ligand-binding domain connected through a segment in the plasma membrane to an intracellular tyrosine kinase domain. In general, RTKs are activated by ligand-induced oligomerization and tyrosine autophosphorylation of specific intracellular substrates such as PLCγ; PI3 kinase, ras, and raf/MEK/Erk1. Tyrosine kinase activity is an absolute requirement for signal transduction through this class of receptor.
The Trk family of RTKs is frequently expressed in lung, breast, pancreatic and prostate cancers as well as in certain type of acute myelogenous leukemia and congenital fibrosarcoma. The tyrosine kinase activity of Trk is believed to promote the unregulated activation of cell proliferation machinery. It is believed that inhibitors of either TrkA, TrkB, or TrkC kinases, individually or in combination, have utility against some of the most common cancers such as brain, melanoma, multiple myeloma, squamous cell, bladder, gastric, pancreatic, breast, head, neck, esophageal, prostate, colorectal, lung, renal, ovarian, gynecological, thyroid cancer, and certain type of hematological malignancies.